Influence of Assist Ion Bombardment on Mechanical Loss of Amorphous Tantala Thin Films for Gravitational Wave Interferometers*

Thermal noise in high-reflectivity coatings is critical for improving the sensitivity of future gravitational wave detectors. Amorphous tantala (Ta₂O₅) thin film, used as high-index material, with lower mechanical loss is expected as one solution to further reduce the coating thermal noise. The material is also regarded as promising candidate for use in resistance random access memory as well as optical applications. In this work, amorphous tantala thin films have been deposited by reactive ion beam sputtering with assist Ar⁺ and Xe⁺ ion bombardment to investigate the effect of ion-induced surface diffusion on coating's mechanical and structural properties. The results show that the atomic structure of ion bombarded tantala thin films remain amorphous and their composition is mostly stoichiometric. The mechanical loss of ion-bombarded tantala coatings shows no significant improvement compared to non-bombarded samples. The coating loss angle of tantala is insensitive to ion dose or ion mass. An analysis based on surface diffusivity suggests that the ion-assisted surface diffusivity may be insignificantly modified under the current typical deposition conditions.